R0, pronounced “R naught,” is a mathematical term that indicates how contagious an infectious disease is. It’s also referred to as the reproduction number. As an infection is transmitted to new people, it reproduces itself.

R0 tells you the average number of people who will contract a contagious disease from one person with that disease. It specifically applies to a population of people who were previously free of infection and haven’t been vaccinated.

For example, if a disease has an R0 of 18, a person who has the disease will transmit it to an average of 18 other people. That replication will continue if no one has been vaccinated against the disease or is already immune to it in their community.

Three possibilities exist for the potential transmission or decline of a disease, depending on its R0 value:

  • If R0 is less than 1, each existing infection causes less than one new infection. In this case, the disease will decline and eventually die out.
  • If R0 equals 1, each existing infection causes one new infection. The disease will stay alive and stable, but there won’t be an outbreak or an epidemic.
  • If R0 is more than 1, each existing infection causes more than one new infection. The disease will be transmitted between people, and there may be an outbreak or epidemic.

Importantly, a disease’s R0 value only applies when everyone in a population is completely vulnerable to the disease. This means:

  • no one has been vaccinated
  • no one has had the disease before
  • there’s no way to control the spread of the disease

This combination of conditions is rare nowadays thanks to advances in medicine. Many diseases that were deadly in the past can now be contained and sometimes cured.

For example, in 1918 there was a worldwide outbreak of the swine flu that killed 50 million people. According to a review article published in BMC Medicine, the R0 value of the 1918 pandemic was estimated to be between 1.4 and 2.8.

But when the swine flu, or H1N1 virus, came back in 2009, its R0 value was between 1.4 and 1.6, report researchers in the journal Science. The existence of vaccines and antiviral drugs made the 2009 outbreak much less deadly.

The R0 for COVID-19 is a median of 5.7, according to a study published online in Emerging Infectious Diseases. That’s about double an earlier R0 estimate of 2.2 to 2.7

The 5.7 means that one person with COVID-19 can potentially transmit the coronavirus to 5 to 6 people, rather than the 2 to 3 researchers originally thought.

Researchers calculated the new number based on data from the original outbreak in Wuhan, China. They used parameters like the virus incubation period (4.2 days) — how much time elapsed from when people were exposed to the virus and when they started to show symptoms.

The researchers estimated a doubling time of 2 to 3 days, which is much faster than earlier estimates of 6 to 7 days. The doubling time is how long it takes for the number of coronavirus cases, hospitalizations, and deaths to double. The shorter the time, the faster the disease is spreading.

With an R0 of 5.7, at least 82 percent of the population needs to be immune to COVID-19 to stop its transmission through vaccination and herd immunity.

The study authors say active surveillance, tracking the contacts of people who contracted the coronavirus, quarantine, and strong physical distancing measures are needed to stop the coronavirus from being transmitted.

The following factors are taken into account to calculate the R0 of a disease:

Infectious period

Some diseases are contagious for longer periods than others.

For example, according to the Centers for Disease Control and Prevention (CDC), adults with the flu are typically contagious for up to 8 days. Children can be contagious for longer than that.

The longer the infectious period of a disease, the more likely a person who has it can transmit the disease to other people. A long period of infectiousness will contribute to a higher R0 value.

Contact rate

If a person who has with a contagious disease comes into contact with many people who aren’t infected or vaccinated, the disease will be transmitted more quickly.

If that person remains at home, in a hospital, or otherwise quarantined while they’re contagious, the disease will be transmitted more slowly. A high contact rate will contribute to a higher R0 value.

Mode of transmission

The diseases that are transmitted the fastest and easiest are the ones that can travel through the air, such as the flu or measles.

Physical contact with a person who has such a disease isn’t needed to transmit it. You can contract the flu from breathing near someone who has the flu, even if you never touch them.

In contrast, diseases that are transmitted through bodily fluids, such as Ebola or HIV, aren’t as easy to contract or transmit. This is because you need to come into contact with infected blood, saliva, or other bodily fluids to contract them.

Airborne illnesses tend to have a higher R0 value than those spread through direct contact.

R0 can be used to measure any contagious disease that may spread in a susceptible population. Some of the most highly contagious conditions are measles and the common flu. More serious conditions, such as Ebola and HIV, spread less easily between people.

This illustration shows some commonly known diseases and their estimated R0 values.

R0 is a useful calculation for predicting and controlling the transmission of disease. Medical science continues to advance. Researchers are discovering new cures for different conditions, but contagious diseases aren’t going to disappear anytime soon.

Take these steps to help prevent the transmission of contagious diseases:

  • Learn how different contagious diseases are transmitted.
  • Ask your doctor about steps you can take to stop the transmission of infection. For example, wash your hands regularly with soap and water, especially before you prepare or eat food.
  • Stay up to date on routine vaccinations.
  • Ask your doctor what diseases you should be vaccinated against.